AnCA molecular crystal has a P2₁/n symmetry with a = 0.39 nm, b = 0.94 nm, and c = 2.9 nm, (see Figure 1 for the structure) [31]. This crystal structure can be rationalized as molecular dimer planes stacked along the [100] direction. Inside each plane, coplanar AnCA dimers are organized into herringbone configuration with anthracene cores parallel to the (100) plane. Despite its simple crystal structure, AnCA self-assembly on Ag(111) displays versatile phase behaviors: Five adlayer structures (4 spontaneously formed and 1 annealed) were observed with distinct molecular surface density.

For the four spontaneously formed AnCA structures on Ag(111) as well as the bulk crystal, the primary building block is AnCA dimer formed through hydrogen bonding of two AnCA molecules. Our DFT calculations suggest an isolated AnCA molecule bears prominent electrical dipole and quadrupole moments, 1.9 D and 22 D·Å, respectively. After the formation of dimer, the molecular dipole moment is mostly cancelled out while the total quadrupole moment is enhanced, and two AnCA molecules can be stabilized through two O···H–O hydrogen bonds with an energy gain of 29 kJ/mol. The dihedral angle (DA) between two anthracene cores in an isolated AnCA dimer is 79° and the length of the hydrogen bond is 1.67 Å. In bulk crystal, the π–π interactions between AnCA molecules in neighboring (100) planes provide strong stabilization energy, which is maximized by stacking dimers parallel along [100] direction. Such kind of crystal structure is popular in aromatic molecules [32]. However, these stacking π–π intermolecular interactions are absent when AnCA dimers adsorbed on Ag substrate (this may presumably contribute to the spontaneous desorption process of AnCA), leading to varied DA depending on surface molecular density. The observed structural diversity on Ag thus stems from the competition of different interactions of AnCA molecules on metal surface, including intermolecular and molecular-substrate interactions, as well as the steric demand due to different molecular surface densities.

The surface structure evolves with time on Ag(111) surface. Phase I is the dominant structure on surface within two hours after the deposition, Phase II is prevailing within 10 hours, and then Phase III within 24 hours. Phase IV, the most stable one, is the only structure on the surface several days later. The phase transition processes were identified during STM measurements (supplementary material). If we leave a freshly deposited sample in vacuum and take STM measurements only at the specific times mentioned above, corresponding phase would prevail on the surface. The structural evolution observed in the experiment is thus attributed to a spontaneous process along with AnCA desorption.

Table 1 summarizes the four spontaneous AnCA self-assembly structures as well as the annealed one. During the experiments, three phase transitions were observed. These phase transitions occur with a decrease in surface molecular density, indicating a structural instability for at least the first three phases. We noted that ordered structures are difficult to form if the initially deposited AnCA is less than 0.7 ML, suggesting a high mobility for AnCA on Ag surface. The high molecular nucleation density could be attributed to a dominant substrate-mediated repulsive intermolecular interaction as reported in SnPc/Ag(111) system [15].

It is difficult to suggest molecular packing models for Phase I and II due to the limited STM resolution. Tentatively, we assign the bright feature in Figure 2b,c to one AnCA dimer based on the following considerations. From the perspective of surface molecular density, the assignment of the feature to a single molecule would result in a density (Phase I, 1 AnCA per 22 Ag atoms) less than those of Phase III (1 AnCA per 15.4 Ag atoms) and Phase IV (1 AnCA per 17.6 Ag atoms), which contradicts the molecular desorption process we have observed in the experiments. Our DFT calculation suggests an AnCA dimer in the gas phase bear a substantial twist angle of 79° between two anthracene cores. A tilted orientation of the constituting AnCA molecules on Ag is thus expected, leading to high molecular packing densities. Our previous studies of ACA also suggested a tilted molecular orientation on Ag(111) for both the chain phase and the dimer phase [18]. The assignment of the feature to a dimer leads to one AnCA molecule per 11 Ag atoms in Phase I, which is comparable to ACA/Ag(111) system.

The AnCA dimers in Phase I are uniformly spaced along the molecular belt direction but unequally spaced across the belt, as shown in Figure 2b. The two center rows inside the unit cell are closer to each other (forming butterfly-shaped features) and the side rows are equally spaced between the center rows. Although the exact packing configuration cannot be determined, it is plausible to think that AnCA dimers in each row adopt different tilt configurations with respect to Ag surface considering the relatively small area to accommodate an AnCA dimer. The different spacings between the dimers across the molecular belt direction render different couplings between the anthracene cores of adjacent dimer rows. This picture can be rationalized as following: inside the unit cell, a favorable coupling configuration between two center dimer rows (with shorter distance in between) may lead to unfavorable coupling configuration of the side dimer rows, resulting in bigger spacing for the side dimer rows. Similar situation was observed elsewhere [33].

The dimer arrangement characters in Phase I are reserved in Phase II. However, the original straight belt structure is broken into a zigzag structure. In Phase II, half of the molecules form building blocks with the same direction as the original Phase I and assemble into one belt species. The other half form building blocks mirror symmetric with respect to the original one about Ag[11̄0] direction and assemble into another belt species. During this phase transition process, the surface molecular density is decreased and 2d molecular gas (noisy areas) is introduced between neighboring molecular belts. These gas phase molecules increase the entropy of the system and lower the system energy, thus effectively reduce the stress presented in the film. The large unit cells as well as the long range ordering suggest an intricate interaction scenario. Possible factors causing these periodical packing faults include Ag surface effects, high order multipole dispersion forces, and molecular steric demand due to high surface density.

It is well known that on close packed metal surfaces (hcp(0001) and fcc(111)), strain-induced structures can cause misfit dislocations. Such dislocations in elemental systems usually generate two types of structures: striped patterns and triangular patterns [34,35]. Among the stripe patterns, Au(111) herringbone reconstruction is intensively studied [36,37]. For molecular self-assembled films, such strain relief processes were also observed. For example, the self-assembled structure of chlorine zinc phthalocyanine (ZnPcCl₈) on Ag(111) shows stripe patterns with periodical faults due to the mismatch between the equilibrium molecular unit cell and Ag substrate [38], and titanyl phthalocyanine (TiOPc) molecules on Ag(111) assemble into periodical triangular pattern when deposition flux is high [20]. Both of them are kinetically accessed and thermally metastable structures. For AnCA, the sequential formation and breakdown of Phase I and II may similarly reflect a strain release process. One common thing for ZnPcCl₈, TiOPc, and AnCA on Ag(111) is the relatively strong intermolecular interaction at high surface density, which is comparable to or even stronger than the adsorbate-substrate interaction. At highest surface density, AnCA molecules form Phase I. The stress, partially relieved through uneven dimer spacing across the belt direction, is presented along the belt direction. This structure is energetically unstable and consecutively breaks down along the belt direction (dotted green line in Figure 2b,d) into AnCA Phase II by introducing periodical packing faults. Details of this phase transition need to be explored by a low temperature STM measurement as well as theoretic calculations.

The coverage dependent AnCA growth experiments show that no ordered structure can be observed if the initial AnCA coverage is lower than 0.7 ML. Pure Phase IV and Phase III structures can be prepared on Ag surface with ca. 0.7 ML and 0.8 ML AnCA coverage, respectively. However, we do not observe the reversed phase transitions by introducing extra AnCA molecules to the already formed low-density phases. This result further corroborates that the high-density phases (Phase I and II) are metastable structures and can only be accessed kinetically.

In thermodynamically metastable Phase I and II, AnCA molecules are highly tilted on the surface to accommodate the high molecular surface density. The intermolecular interaction is dominant in these phases. However, they are kinetically trapped structures and not stable at room temperature. As partial AnCA molecules desorb from the surface, interaction between AnCA and Ag substrate becomes dominant. Molecules tend to occupy more space on Ag substrate to gain more adsorption energy. Compared with Phase I, the packing densities for Phase III and Phase IV decrease by 28% and 37%, respectively. The building units (AnCA dimers) are now clearly resolved with STM measurements for low density phases. The reduced surface density and improved STM resolution suggest smaller tilting angles of molecules on Ag surface. A larger overlapping between conjugate π electrons of the anthracene core and metal surface is thus expected, leading to the commensurate Phase III and IV and improved structural stability of AnCA on Ag surface. The annealed kagome structure (Phase V) provides a further proof of this argument. In this phase, the carboxyl groups deprotonate during annealing [39,40], and weak hydrogen bonds form between oxygen atoms and ring hydrogen atoms (O···H–C). The flat-lying molecular configuration yields the lowest surface density, ensures the strongest AnCA-Ag interaction, and results in the best STM resolution.

It is interesting to compare the self-assembly structures of ACA and AnCA on Ag(111). Only one ACA dimer phase was observed on the open Ag terrace while four distinct AnCA dimer phases (Phase I–IV) were observed. Previous studies suggested that when heterocyclic molecules adsorbed on metal surfaces, the orientation of the adsorbate reflects a balance between π-substrate bonding and σ-substrate bonding [41,42]. As coverage increases, the ring plane tilts up from the surface to enhance the interaction between the lone pair electrons and the substrate. In ACA dimer phase, this enhanced interaction between nitrogen lone pair electrons and Ag surface at high coverage would impede the twist flexibility of individual molecule inside ACA dimer and result in a single dimer phase. In contrast, AnCA dimers (without such effect) can twist flexibly with increasing coverage and result in a series of dimer phases with distinct coverage. In addition, as suggested in the reflection adsorption infrared spectroscopy measurements (RAIRS), AnCA molecules are bound into dimers on Ag surface at very low coverage. Increasing AnCA coverage, the dimer configuration is maintained with an increasing molecular tilt angle on Ag surface [43]. ACA molecules, however, are preferentially linked by head-to-tail hydrogen bonds (O–H···N) at low and intermediate coverage, forming the chain phase. Only at relatively high coverage, ACA dimers emerge due to the steric effect.